To produce holes that extend entirely through a layer of material, it must be ensured that the laser beam is switched off only after the material in the area of the hole has been completely ablated. Due to fluctuations in the process parameters, for example, laser power and focus position, or in the properties of the layer of the material, for example, fluctuations in the density of the material or deformations, it is not possible to ensure this type of complete ablation of material by setting a fixed working time or pulse sequence, so it is common practice to monitor the ablation of the material and to switch off the laser immediately after each hole has been completed, thereby ensuring that additional laser power is not generated unnecessarily, which could potentially damage other layers, and that the diameter of the hole on the exit surface is kept small.
In procedures in which the laser beam as a tool is directed in a permanently fixed position onto the surface of the workpiece and the workpiece is moved relative to the laser beam to produce a series of holes at certain intervals from one another in the workpiece, the entire ablation of the material can be monitored, e.g., by means of a sensor which is disposed in a permanently fixed position on the surface of the workpiece facing away from the laser beam and which detects the exiting laser beam.
In contrast, in procedures in which the workpiece is mounted in a permanently fixed position and the tool, i.e., the laser beam, is moved relative to the workpiece, monitoring by means of a sensor that is disposed on the surface facing away from the laser beam is not useful. Since the relation between the position of the sensor and the position of the laser beam must always be fixed, the sensor and the laser beam must be moved together relative to the workpiece. This entails a greater degree of complexity in terms of construction and control technology and, consequently, a higher susceptibility to breakdowns.
Another disadvantage of disposing a sensor on the surface facing away from the laser beam is that such configurations and methods associated with such configurations cannot be used when the workpiece is a multilayer structure and the holes are to be produced only in the first layer of the material.
In addition to monitoring the ablation of material for the purpose of switching off the laser beam after a hole has been completed, certain applications in which the workpieces are to be used make it necessary to detect a quality parameter for the hole while the hole is being produced, which quality parameter makes it possible to decide whether the workpiece produced with the series of holes is to be classified as conforming or nonconforming.
In particular, when the material layer is a layer of an airbag cover and the series of holes form a predetermined rupture line, it is especially important for the diameters of the holes to be within a narrow tolerance range. The hole diameters and the hole spacings as geometric parameters and the bursting strength of the material are jointly responsible for the tear resistance of the predetermined rupture line, which is why the focus position must be kept stable throughout the process.
Since an airbag cover is a safety-relevant component, the predetermined rupture line of which must reliably burst open at a defined bursting force when an airbag disposed behind the airbag cover is activated, automobile manufacturers as a rule are not satisfied with proof of the technical stabilization of process parameters, but demand 100% quality control and quality documentation which, for reasons of efficiency, have to be performed during the production process.